A: Most experts agree that driving in a relatively fuel-efficient car (25 to 30 miles per gallon) usually generates fewer greenhouse-gas emissions per person per mile than flying does – and it only gets better when you carpool.

In assessing the global-warming impact of a trip from Philadelphia to Boston (about 300 miles), the environmental news website Grist.org calculates that driving would generate some 230 pounds of carbon dioxide (CO2) – the leading greenhouse gas – per typical medium-sized car, regardless of the number of passengers. Flying on a commercial jet, however, would produce some 400 pounds of CO2 per passenger.

What this also means, of course, is that while even driving alone is slightly better from the standpoint of greenhouse-gas emissions, carpooling makes environmental sense. Four people sharing a car collectively would generate 230 pounds of CO2, while the same four people taking up four seats on a plane would generate some 1,600 pounds.

Journalist Pablo Päster of Salon.comextends the comparison further to a cross-country trip, and comes to similar conclusions. (Differences in the math are attributable to the use of varying assumptions regarding fuel usage and source equations.) His conclusion: Flying from San Francisco to Boston, for example, would generate nearly 1-1/2 tons (2,900 pounds) of greenhouse gases per passenger, while driving would account for one ton per vehicle. Sharing the drive with one or more people would lower each individual’s carbon footprint accordingly.

But just because driving might be greener than flying doesn’t mean it always makes the most sense. With current gas prices, it would cost far more in fuel to drive across the United States in a car than to fly nonstop coast-to-coast. And that’s not including the expense of meals and hotels along the way. You can consult AAA’s nifty online Fuel Cost Calculator, where you can enter your starting city and destination along with the year, make, and model of your car to get an accurate estimate of what filling ‘er up will cost between points A and B.

Once you’ve made your decision to drive or fly, consider purchasing carbon offsets to balance out the emissions you are generating with cash for renewable energy development. TerraPass, among others, makes it easy to calculate your carbon footprint based on how much you drive and fly (as well as home energy consumption), and then will sell you offsets accordingly.

(Monies generated through carbon offsets fund alternative energy and other projects, such as wind farms, that will ultimately take a bite out of or eliminate greenhouse-gas emissions).

Of course, an individual’s emissions from riding a bus (the ultimate carpool) or a train would be significantly lower. Mr. Päster adds that a cross-country train trip would generate about half the greenhouse-gas emissions of driving a car. The only way to travel greener might be to bicycle or walk – but the trip is long enough as it is.